The polyphenylene ether resin is known as a high performance engineering thermoplastic. However, the resin has relatively high melt viscosities and softening points as well as small impact strength so that it is commercially unattractive to melt-mold the resin alone because of the required high temperatures needed to soften the resin and the problems associated therewith such as discoloration or oxidative degradation.
To solve the above problems, U.S. Pat. No. 3,383,435 discloses a resin composition composed of a polyphenylene ether resin and a rubber modified, impact resistant polystyrene such as a styrene-acrylonitrile copolymer or a styrene-acrylonitrile-butadiene copolymer. This resin composition has an improved moldability to an extent, but is still inferior in impact strength.
A resin composition is also known which contains an elastomer or rubber in addition to a polyphenylene ether resin and rubber modified polystyrene so that the resin composition has an improved impact strength. For example, there is disclosed in British Patent No. 1,477,706, a resin composition composed of a polyphenylene ether resin, a rubber modified polystyrene and a hydrogenated block copolymer of the A-B-A type wherein A is a polymerized monoalkenyl aromatic hydrocarbon block and B is a polymerized conjugated dienic hydrocarbon block in which the unsaturation of the block has been reduced to less than 10% of the original unsaturation. This resin composition also has an improved impact strength to an extent, but is still insufficient for practical application. Moreover, the resin composition has an insufficient flow.
As a generally accepted method of improving impact strength of a resin, it is known to incorporate a core-shell polymer which has, as well known in the art, a core of an elastomer and a shell of glassy polymer, into a resin as an impact modifier. The core-shell polymer is melblended with a resin, and has an advantage that it is dispersible in a resin substantially irrespectively of the conditions under which the core-shell polymer is melt-blended with the resin. However, no core-shell polymer has hitherto been known which satisfactorily improves impact strength of the polyphenylene ether resin.
For instance, a core-shell polymer having a shell composed of polymers of methacrylic acid esters and/or acrylic acid esters fails to impart satisfactory impact strength to a polyphenylene ether resin.
In turn, a polyphenylene ether resin composition is also known which contains, as an impact modifier, a core-shell polymer having a shell of aromatic vinyl polymer, as disclosed in U.S. Pat. No. 4,684,696. The core-shell polymer is allegedly composed of an acrylate core surrounded and interpenetrated by a crosslinked styrenic shell. However, it has been found out by the present inventors that the emulsion polymerization of styrene in the presence of crosslinked acrylate rubber particles in accordance with the description in the reference provides, as a matter of fact, two-phase polymer particles in which the resultant styrenic polymer is dispersed in the acrylate rubber phase to form a salami-like structure. More specifically, the resultant polymer particles have substantially no styrenic polymer on the surface, so that the polymer particles are very adhesive to each other and thus very difficult in handling. And yet the polymer fails to impart sufficient impact strength to a polyphenylene ether resin. When an increased amount of styrene is used in the production of the two-phase polymer so that the resultant polymer particles have styrenic polymers on the surface, the resultant polymer particles are useless as an impact modifier.